In the telephone industry it has long been standard practice to provide multiple line service for a plurality of telephone instruments by use of key telephones which are interconnected through a key service unit or KSU that provides the central intelligence for the system. In such systems, all of the telephone instruments must interface through the key service unit, and several major disadvantages thus arise.
First, all of the telephone instruments in a conventional key system must be wired directly back to the key service unit in a mode of installation called home run wiring. This requirement and the large number of conductors necessary to make the connections for each telephone, significantly complicates system installation and escalates cost. Installation cost and complexity multiply rapidly with increases in the number of lines the system supports due to the fact that additional home run wiring must be provided for each additional line to serve each telephone instrument in the system. Home run installation thus is expensive, complicated, and difficult to manage when considering options for system expansion.
A second disadvantage of the conventional key telephone system is that it inevitably requires a key service unit in addition to the plural telephone instruments. The KSU thus represents an incremental (and often fairly large) cost factor which increases the per-telephone cost of the system. This is an especially troublesome drawback in multiple line systems with only few telephone instruments since the KSU cost must be prorated over a very few telephones.
Among the issued patents dealing generally with telephone systems of the type above described are the following. U.S. Pat. No. 4,410,767 discloses a KSU telephone system in which system operation and control relies on individually predetermined different frequencies of respective telephone sets.
U.S. Pat. No. 4,024,353 discloses a KSU type key telephone system which supports only a single incoming line. U.S. Pat. No. 4,280,022 discloses a KSU type system which is wired in a star configuration. U.S. Pat. No. 4,185,173 discloses a KSU type system in which indicator lamp status information and voice signals are multiplexed to the station sets using differential transformer techniques. U.S. Pat. No. 4,394,544 discloses a non-multiplexed, bused, lamp interconnect scheme in a telephone system which requires a separate conductor for each indicator lamp to be controlled.
U.S. Pat. No. 4,093,830 discloses a conventional KSU type telephone system. U.S. Pat. No. 4,079,212 discloses an extension telephone system which, although it requires a KSU, does not support multiple lines. U.S. Pat. No. 4,004,106 discloses operation of a line circuit for use in a KSU. U.S. Pat. No. 4,013,844 discloses a KSU type telephone system incorporating a double coil relay for control of line circuit operation. U.S. Pat. No. 4,064,373 also discloses a line circuit configuration, as does U.S. Pat. No. 4,101,741. U.S. Pat. No. 4,323,734 discloses circuitry for implementing control signal information on the tip and ring pair of a telephone system by line polarity reversals and DC level changes. U.S. Pat. No. 4,408,102 discloses telephone control circuitry residing in a central PBX or KSU, with signaling based on tone signaling. U.S. Pat. No. 4,382,163 discloses use of a microcomputer to detect and control traditional line circuit functions for station set control.
Other U.S. Pat. Nos. relating to telephone systems generally are: 4,382,163; 4,207,439; 4,374,308; 4,080,517; 4,029,909; and 4,260,857.